1. Field of the Invention
The present invention relates to a focus detection apparatus for performing automatic focus adjustment control effectively used with an imaging apparatus such as a video camera.
2. Description of the Related Art
Generally, the contrast automatic focus (AF) system is employed as an AF system provided to an imaging apparatus such as a video camera. According to the contrast AF system, a high-frequency component is extracted from a video signal acquired by using an image sensor, and a so-called AF evaluation value signal is generated as a signal indicating a focusing state. Then, the position of a focus lens is controlled so as to maximize the AF evaluation value signal.
Further, there has been discussed an imaging apparatus employing the so-called hybrid AF system which is a combination of the contrast AF system and the so-called Non-TTL (Through the Lens) phase difference detection (Non-TTL AF) system in Japanese Patent Application Laid-Open No. 2002-258147.
According to the Non-TTL AF system, a distance to an object is directly detected by an AF sensor with use of light that does not pass through an imaging optical system, and the position of the focus lens is controlled based on the detected distance. Employing the hybrid AF system, which is a combination of the Non-TTL AF system and the above-described contract AF system, can realize an imaging apparatus having both a highly accurate and highly resolving focusing performance derived from the contrast AF system, and a high-speed focusing performance derived from the Non-TTL AF system.
According to the Non-TTL AF system, a correlation between two images is calculated based on signals of the object images acquired from a pair of line sensors, and a phase shift amount is calculated therebetween. Then, the distance to the object is measured based on the triangulation principle, and the position of the focus lens is controlled based on the detected distance.
If distances to objects are constant, the phase shift amounts of the objects detected by the line sensors are also uniquely determined. However, when there is a conflict among a plurality of distances (also referred to as “perspective conflict”), such as simultaneous entries of an object positioned 1 m from the imaging apparatus and an object positioned 10 m from the imaging apparatus into the angle of view of the sensors, it is difficult to uniquely determine the phase shift amounts of the objects.
To solve this problem, for example, Japanese Patent Application Laid-Open No. 8-15603 discusses a method of, in a case where it is determined that there is a conflict among a plurality of distances, dividing an image into a plurality of blocks, recalculating a correlation in each of the blocks, and respectively calculating the plurality of distances.
However, according to this method, an image is divided into small blocks in a manner predetermined to some degree, and therefore even recalculating a correlation in the small block may still result in occurrence of a conflicting state among a plurality of distances, thereby failing to perform focus detection with high precision. In this case, a correlation should be calculated once more with use of adjacent small blocks. In this way, this method lacks real-time responsiveness, which is a necessary feature for a built-in device, and fails to satisfy sufficient accuracy.